The present invention generally relates to optical networks and more particularly, to a method for modulating an optical communications signal.
A Mach-Zehnder modulator (MZM) is a device that is commonly used to produce a modulated optical carrier in an optical network. The MZM can modulate the intensity of a light signal with a voltage drive signal. Typically, an MZM includes two phase modulator arms and a splitter, such as a Y-junction splitter, at the front end to split a continuous-wave (CW) light signal between the two phase modulator arms. A combiner, such as a Y-junction combiner, is coupled to the other end of the two phase modulator arms to combine the phase shifted light signals. If the light signals are in phase when recombined in the Y-junction combiner, they are coupled into a single mode output waveguide. If the light signals are out of phase when recombined in the Y-junction combiner, they are transformed into a higher order mode and lost into the substrate of the MZM.
The voltage drive signal that controls the phase shift of the CW light signal is supplied to either a single electrode attached to one arm of the MZM for single arm drive or supplied to electrodes attached to both arms of the MZM for dual arm drive. Because the MZM modulates the intensity of the CW light signal with a voltage drive signal to produce a modulated optical signal, the greater the light intensity ratio between the xe2x80x9conxe2x80x9d state of the modulated optical signal and the xe2x80x9coffxe2x80x9d state of the modulated optical signal results in a modulated optical signal with fewer bit errors. This light intensity ratio is known in the art as the extinction ratio (ER) and is the measure of the modulated optical signal""s light intensity against the background noise. As a result, the generation of a high ER produces a higher quality optical signal that permits a greater span between repeaters in the optical network.
The present invention provides an approach to improve the bit error rate of a modulated optical signal over positive dispersive fiber by generating a modulated optical signal with a less than maximum ER.
In one embodiment of the present invention, a method for processing an optical signal in an optical communications network is practiced. The optical signal is input into an optical modulator and the optical modulator is over-driven with a modulation voltage signal. Consequently, the overdriving of the optical modulator results in a reduced ER that yields an improved bit error rate at a receiver.
The above-described approach benefits an optical communication network in that the quality of service can be improved for all network customers by overdriving an optical modulator. Moreover, the overdriving of the optical modulator with the modulation voltage signal reduces the bit error rate of the modulated signal as received at the end of a single mode fiber conductor.
In accordance with another aspect of the present invention, a method is performed to improve the bit error rate of an optical signal transmitted over an optical transmission medium. By providing a stabilized light beam to an optical modulator and overdriving the modulator with a voltage signal, the optical signal""s extinction ratio at the receiver is reduced, but the bit error rate of the optical signal is improved. In one embodiment, the optical modulator is a semiconductor Mach-Zehnder modulator and in another embodiment, the optical modulator is a lithium niobate Mach-Zehnder modulator.
The above-described approach allows the use of an off-the-shelf Mach-Zehnder modulator to produce an improvement in an optical signal""s bit error rate. In this manner, an optical network provider can improve the network""s quality of service with minimal disruption to service. The time to implement the improvement is minimal, as is the cost to implement the improvement. Furthermore, the improvement can be realized without adding complexity to the software and hardware that control the optical modulator. As a result, a significant improvement in an optical signal bit error rate can be attained with a minimal impact to existing optical network hardware and software.